A problem with operating hand portable wireless devices having small planar antennas, such as planar inverted-F antennas (PIFAs), is that when a user holds a device, the antenna's impedance changes predominantly reactively. As a result the matching of the antenna to radio frequency circuitry is affected adversely by these reactive changes.
EP 1 564 896 A1 discloses altering the value of an impedance connected between a power amplifier and an antenna to achieve power control in the output stage of a power amplifier. In operation the actual load impedance at the antenna is measured and the value of the impedance is adjusted so that only a purely resistive load is experienced by the power amplifier.
WO 2006/054246 discloses a controlled matching stage connected between the output of a power amplifier and an antenna stage. The controlled matching stage comprises a phase detector for detecting the phase difference between a first signal derived from the power amplifier and a second signal derived from an input to a switching stage coupled to the antenna stage. The difference in phase between the first and second signals is used to adjust the impedance of the switching stage. Typically the switching stage comprises a series LC circuit comprising a fixed inductance and an adjustable capacitance.
GB 0 804,103A discloses an automatic tuning system using servo motors driving respectively an adjustable antenna input coupling and a sliding short circuit. The resistance and reactance of a transmission line connecting a transmitter to an antenna are sensed and the results are used in driving servo amplifiers controlling the servo motors. Measures are disclosed enabling the servo motors initially to be driven rapidly and then to move more slowly.
GB1 362 154 A discloses an automatic tuner for transforming the impedance of an antenna to a load resistance required for the power amplifier output stage of a transmitter. The automatic tuner uses a method of control of the tuning circuit element requiring phase and impedance inputs indicative of the reactive condition of the selected antenna. The phase input is used to control the switching of capacitors in an antenna impedance matching network and the impedance input is used to control the switching of impedances in the antenna impedance matching network.
WO 2006/038167 A1 discloses coupling a RF power amplifier to an antenna by way of a circuit for detecting the impedance of the antenna. The circuit detects a signal travelling from the RF power amplifier to the antenna and measures the peak current of the signal. More particularly the circuit comprises first means for sensing the peak value of the output voltage of the RF power amplifier, second means for sensing the peak of the output current of the RF power amplifier, and third means for deriving the phase between the output voltage and output current.
WO 2004/010595 A1 discloses a device for dynamic impedance matching between a power amplifier and an antenna. The device includes a circulator which routes a signal received from the power amplifier at a first port via a second port to the antenna. Additionally the circulator diverts a signal reflected at the antenna and received at the second port through a third port. A matching network is provided. In operation a directional coupler diverts a proportion of the signal travelling from the power amplifier to the antenna, from which the magnitude and phase of the signal may be derived, to a signal detector. The circulator routes the entire signal reflected at the antenna into the signal detector. The signal detector passes the magnitude and phase of both the signal travelling to the antenna and the signal reflected at the antenna to a controller, which evaluates the information received from the signal detector in order to determine the present impedance value of the antenna and to correct the controllable matching network containing active and passive components in accordance with the determined impedance value of the antenna.